Liquid crystal displays have a structure in which a polarizing film is arranged on both sides of a liquid crystal cell and, in addition to the polarizing films, a variety of optical films such as a phase difference film, a viewing angle expansion film and a brightness-improving film are also used in liquid crystal panels.
Conventionally, cellulose-based resins have been widely used as optical films from the standpoints of their lamination with polyvinyl alcohols used for polarizing films, transparency, toughness and the like.
In recent years, due to accelerated reductions in the thickness and weight of televisions and laptop computers, there is a demand for further thickness reduction and multifunctionalization of optical films as well. However, cellulose-based resin films themselves do not have sufficient retardation (birefringence or optical properties) essential for optical films; therefore, various methods for imparting retardation to a cellulose-based resin film have been examined.
For example, a method of adding a retardation-increasing agent is known; however, when an optical film is reduced in thickness, since the retardation value is dependent on the thickness, in order to allow a conventional retardation-increasing agent to exhibit its performance, the retardation-increasing agent is required to be added in a large amount.
This causes problems that, for example, the retardation-increasing agent bleeds out from the film surface and a non-uniform film is consequently produced; therefore, it is extremely difficult to prevent deterioration of the working efficiency and productivity and to satisfy diverse performance requirements that conform to such thickness reduction. Furthermore, although methods of reducing the total acyl group substitution degree of a cellulose resin and the like have been proposed, these methods are not satisfactory from the practical standpoint because they present new problems such as an increase in moisture absorption.
Under these circumstances, there have been recently proposed methods of adding a specific retardation-increasing agent for practical use. For instance, Patent Document 1 proposes a phase difference plate which is composed of a cellulose ester film that contains a retardation-improving agent comprising a compound having a 1,3,5-triazine ring. Further, in Patent Document 2, a carbamic acid ester compound is proposed as a retardation-controlling agent.